The present invention relates generally to rotational control apparatus and, more particularly, to an apparatus for controlling the transmission of rotational forces between first and second relatively rotatable members, and specifically in the preferred form to a friction clutch.
When it is desired to intermittently transmit rotational forces from one component to another component, it is commonplace to interconnect the components with a clutch and then to selectively activate and deactivate the clutch as needed. Known clutch arrangements can be controlled using electrical, mechanical, pneumatic or hydraulic based actuation systems. Fluid actuated systems typically utilize pressure developed in a piston/cylinder arrangement to apply an axial load on a coupling assembly in order to transmit torque between the rotatable components.
In the art, known fluid actuated clutches have been designed for exclusive use with a particular type of actuation fluid. More specifically, fluid actuated clutches are designed to operate with either a gaseous medium or a liquid medium, but not both. Therefore, a pneumatic actuated clutch cannot operate properly with hydraulic fluid, generally due to the characteristics of pneumatic seals. More specifically, the pneumatic seals would not properly function to prevent the flow of hydraulic fluid there across. Instead, the hydraulic fluid would weep past the pneumatic seals and contaminate the friction interface of the coupling assembly. In addition, typical finishes of pneumatic seal surfaces are generally insufficient for hydraulic use as the hydraulic oil tends to become embedded in the surface imperfections, thereby developing a poor sealing surface. This is particularly true where the piston/cylinder surfaces are cast from aluminum. The surface finish required to seal hydraulic oil is very critical, with rms values of at least eight (8) being required for proper seal life. Although the costs associated with manufacturing cast components is attractive, the cast surfaces cannot be finished to this degree. Therefore, clutch manufacturers must resort to utilizing extruded or milled materials for at least the sealing surface components for hydraulically actuated clutches. Of course, this undesirably adds to the overall cost of the hydraulically actuated clutch.
Commensurate problems are created if gaseous mediums are utilized to actuate a clutch designed for liquid mediums. For instance, seals used for movable components in any fluid actuated clutch require a certain amount of lubrication. A lack of lubrication in the piston/cylinder area results in increased frictional forces, thereby preventing proper actuation of the clutch. In hydraulic oil actuated clutches, the required lubrication is provided by the actuating fluid medium itself, but if a gaseous medium is utilized, the source of lubrication is lost. Therefore, clutches designed for use with hydraulic oil will not be provided with a necessary amount of lubrication if a pneumatic pressure source is utilized.
Based on the above problems, clutch manufacturers must produce different types of clutches depending on the particular actuation fluid. Of course, designing, producing and assembling separate clutch components for pneumatic and hydraulic based systems represent a significant cost which must inevitably be borne by the consumer. Therefore, there exists a need in the art for a rotational control apparatus which is designed for more universal applicability and, more specifically, to a clutch arrangement which can effectively operate as either a liquid or a gaseous based actuation system.
In addition to the above concerns, manufacturers of friction clutches must consider the effect of wear on the long term operability of the clutches. In most friction clutches, the transmission of torque is accomplished by supplying an actuating fluid medium into a pressure chamber to cause shifting of a piston which, in turn, causes engagement of frictional material carrying elements of a coupling assembly. A return spring is generally incorporated which biases the coupling assembly to a disengaged position. Therefore, when the actuating fluid is withdrawn from the pressure chamber, the frictional elements will be automatically spread apart. Most typically, a compression spring is used to bias the frictional elements in this manner.
Unfortunately, compression springs generate a preset amount of force per unit of length over which they are compressed. The ratio of force generated to length compressed is referred to as the spring value (K). With a compression spring, the amount of spring deflection increases, in a linear fashion, with increasing load. When used in a friction clutch, the spring will become increasingly loaded as a function of the axial wear of the friction material. In some situations, the force needed to disengage the coupling assembly is substantial, even up to thirty (30) percent of the required clutch engagement force. In other words, the larger the amount of torque required by the clutch, generally the larger the required disengagement or return force. To generate this large of a return force, clutch springs having high K values must be utilized. However, as the friction material wears, a substantial increase in the load supplied by the springs will develop. This increase in the return force due to the wearing of the friction material reduces the load carrying capacity of the clutch.
Based on the above, there also exists a need in the art for an apparatus for controlling the transmission of rotational forces between multiple rotatable members wherein the return force generated by the apparatus remains constant, regardless of the amount of wear on the is frictional elements. More specifically, there exists a need for a clutch arrangement wherein a constant return force is developed such that the torque capacity of the clutch can be maintained constant over the entire range of deflection.